Tuesday, July 14, 2009

Episode #51: Leapin Lizards! ARP Stuff!

It's Hal's turn in the Bucket:

Mr. Bucket was pointing out the other day that we haven't had much to say about viewing and manipulating the ARP cache from the command line, and that's a darn shame. So let's remedy that little oversight right now.

The ARP cache is the system's local mapping of ethernet addresses to IP addresses of machines on the same LAN. On Unix systems, you can use "arp -a" to dump the cache or just dump information for a particular host:

$ arp -a
gw.deer-run.com ( at 00:04:23:5f:20:98 [ether] on eth0
deer.deer-run.com ( at 00:30:48:7b:22:2e [ether] on eth0
$ arp -a gw
gw.deer-run.com ( at 00:04:23:5f:20:98 [ether] on eth0

As with most network administration commands on Unix, the "-n" flag suppresses the hostname information, in case you're having name resolution problems that are causing the command to hang:

$ arp -an
? ( at 00:04:23:5f:20:98 [ether] on eth0
? ( at 00:30:48:7b:22:2e [ether] on eth0

Occasionally monitoring your ARP cache can be interesting, because it can alert you when system hardware is changing on your network and also to ARP spoofing attacks. Here's a little bit of shell fu to watch for changes to a particular ARP entry:

$ while :; do c=`arp -a | awk '{print $4}'`; \
[ "X$c" == "X$p" ] && echo "$c (OK)" || echo "$p to $c (CHANGED)"; \
p=$c; sleep 5; done

to 00:04:23:5f:20:98 (CHANGED)
00:04:23:5f:20:98 (OK)
00:04:23:5f:20:98 (OK)

Notice the equality test where I'm putting an "X" in front of the value of each variable ("X$c" == "X$p")? That's a common trick for doing comparisons where one of the values might be null-- as in our case when we first enter the loop and $p is not set yet. If we just wrote, "[ $c == $p ]", the shell would generate a syntax error message during the first pass through the loop because it would interpret our fu as "[ 00:04:23:5f:20:98 == ]" which is clearly bogus. The extra "X" ensures that neither side of the comparison is ever empty.

Actually, the above example is a little bogus (though it includes plenty of tasty shell fu), because there's already a tool called Arpwatch that will monitor your ARP cache for changes-- expected or otherwise. I'll often run this tool on a couple of machines on critical networks, just to look for signs of hanky panky.

If you're running with root privileges, you can also use the arp command to manually manipulate your ARP cache. For example, "arp -d" deletes ARP entries. However, since ARP dynamically relearns the deleted entries, they don't tend to stay deleted for long:

# arp -d deer
# arp -a
gw.deer-run.com ( at 00:04:23:5f:20:98 [ether] on eth0
deer.deer-run.com ( at on eth0
# ping -c 1 deer >/dev/null
# arp -a
gw.deer-run.com ( at 00:04:23:5f:20:98 [ether] on eth0
deer.deer-run.com ( at 00:30:48:7b:22:2e [ether] on eth0

The root account can also use "arp -s" to add static ARP assignments to the cache. For example, I was recently debugging a VPN problem for one of my customers and we suspected the Open Source VPN software we were running on our Unix system wasn't properly doing Proxy ARP for the remote clients. So we manually added an ARP entry for the client we were testing with that pointed to the MAC address of our gateway (note that you must do this on the gateway machine that is the owner of the specified MAC address):

# arp -s 00:04:23:5f:20:98 pub
# arp -an
? ( at 00:30:48:7b:22:2e [ether] on eth0
? ( at PERM PUB on eth0

The extra "pub" argument means that the host should "publish" this static ARP entry-- that is, respond with our local MAC address when other hosts on the LAN make ARP requests for In other words, our gateway should perform Proxy ARP for this address.

Since we did not specify the "temp" option in the above command, this static ARP entry will persist until we reboot the machine. If you specify "temp", then the entry will time out as normal when your system flushes its ARP cache. Generally, though, if you're specifying static ARP entries, you're doing it for a reason and you want them to stick around. For example, some highly secure sites will populate static ARP entries for all authorized hosts on their local LANs in order to thwart ARP spoofing attacks (static ARP entries will never be overwritten by dynamic ARP information learned from the network). Of course, the static ARP entries will need to be reloaded every time the system reboots, so you'll need to create a boot script to automatically reload the entries from a configuration file. And you'd need to update the configuration file every time a host was added or a NIC was changed on the LAN. It's a huge hassle and not normally done in practice.

That's about it for ARP on Unix. If memory serves, things aren't too different on Windows, but I bet Ed has some tasty fu for us anyway...

Ed responds to Hal's Bucket Screed:
As Hal anticipated, the arp command on Windows is very similar to the Linux version, but there are a few annoying deltas. To dump the ARP cache, we can run:

C:\> arp -a

Interface: --- 0x10003
Internet Address Physical Address Type 00-0c-29-c2-9f-1b dynamic de-ad-be-ef-00-00 static 00-0c-29-a3-2c-8b dynamic

There are a couple of noteworthy things here: First off, we don't see host names. The Windows arp command works exclusively with IP addresses and not names. Thus, we don't have a -n option, and cannot enter domain names of the machines at command line. If we want to zoom in on individual machines, we need to enter an IP address, not a name:

C:> arp -a

Interface: --- 0x10003
Internet Address Physical Address Type de-ad-be-ef-00-00 static

Next, note that we have that "Type" column, which tells us whether an entry is dynamic or static. This is handy, because we can list all static ARP entries using:

C:\> arp -a | find "static"

Next, note that the Windows arp command uses dashes between octets in the MAC address, and not colons. This is annoying, and I often type it wrong before I type it right.

Hal's ARP-monitoring command is fun and useful, so I tried to mimic its functionality almost identically on Windows, coming up with the following command (brace yourselves):

C:\> cmd.exe /v:on /c "for /l %i in (1,0,2) do @for /f "skip=3 tokens=2" %i in
('arp -a') do @set current=%i & (if X!current!==X!previous!
(echo %i ^(OK^)) else echo !previous! to %i ^(CHANGED^)) & set previous=%i
& ping -n 6 > nul)"

!previous! to 00-0c-29-c2-9f-1b (CHANGED)
00-0c-29-c2-9f-1b (OK)
00-0c-29-c2-9f-1b (OK)
00-0c-29-c2-9f-1b (OK)
00-0c-29-c2-9f-1b (OK)
00-0c-29-c2-9f-1b to 01-02-03-04-05-06 (CHANGED)
01-02-03-04-05-06 (OK)
01-02-03-04-05-06 (OK)
01-02-03-04-05-06 (OK)
To analyze this command, let's work our way from the outside in. I'm invoking a cmd.exe with delayed environment variable expansion so that I can store and compare some variables inside my command. I then have a FOR /L loop that counts from 1 to 2 in steps of zero to keep this puppy running forever. I add a 5-second delay at the end by pinging six times. Now, we get to the core of the command. I run a FOR /F loop to parse the output of my arp command, skipping 3 lines of output to zoom in on what I want, grabbing the second item (the MAC address), which I place into iterator variable %i. At each step through the loop, I set the variable "current" to my iterator variable so I can compare it against my previous MAC address for that ARP table entry with an IF statement. IF statements don't like to deal with FOR loop iterator variables, so I have to move my %i value into "current" for comparison. I then compare it with the previous value, using Hal's little X prepend trick so they are never empty strings. If they match, I print out the MAC address (%i) and the "(OK)" just like Hal does. I have to put ^'s in front of my parens so that echo knows to display them. If the current and previous MAC address from that arp entry do not match, I print out that the previous changed to %i followed by "(CHANGED)". Finally, I store the current value of %i as my "previous" variable and then wrap around. It's not elegant, but it gets the job done, and looks very similar to the output and functionality of Hal's command.

Lessee... what other stuff does Hal have for us?

Yup... we have "arp -d" to delete an ARP entry:

C:\> arp -d

And, we also have arp -s to plant a static ARP entry:

C:\> arp -s 01-02-03-04-05-06

Unfortunately, the Windows arp command does not have a "pub" option like the Linux command to implement proxy ARP functionality.

By the way, this whole discussion of the arp command was triggered by Byte Bucket's comment to Hal and me in our discussion of scheduling tasks. Byte (if I can be so informal) mentioned that on Windows, to define a static ARP entry, Microsoft recommends that you schedule a startup task, by saying, "To create permanent static ARP cache entries, place the appropriate arp commands in a batch file and use Scheduled Tasks to run the batch file at startup." This is a good idea if you ever want to hard code your arp tables to avoid ARP cache-poisoning attacks. Of course, in their recommendation, Microsoft never tells you how to actually do this. Well, never fear... Command Line Kung Fu is here!

We start by creating our batch file for the static ARP entry:

C:\> echo arp -s aa-bb-cc-dd-ee-ff > C:\static_arp.bat

You can add more static arp entries into this file if you'd like, echoing in one arp command per line, appending to the file with >>. I'm storing it right in c:\ itself for easy spotting, although you may find that tucking it away somewhere you keep your admin scripts is a less clutterful approach. Then, we make that a startup task using the schtasks command, which we described in our last episode (#50):

C:\> schtasks /create /sc onstart /tr c:\static_arp.bat /tn static_arp

And there you have it! We've seen how to make a little arp-cache watcher in a single command, as well as how to implement static arp entries at system startup. Good topic, Hal!

UPDATE: Diligent reader Curt points out that in Windows Vista and 7, you can add a static ARP entry using the netsh command, with the following syntax:

C:\> netsh interface ipv4 add neighbors "Local Area Connection" 12-34-56-78-9a-bc

And, I'm delighted to report that this will survive a reboot, so no scheduled task startup script is required. However, this neighbors context for netsh only appears in Vista, 2008, and Win7. In XP or 2003, you can go with the schtasks method I outline above. Thanks for the input, Curt!